Transmissibility and Persistence of the Plasmid-Borne Mobile Colistin Resistance Gene, mcr-1, Harbored in Poultry-Associated E. coli

Abstract

Colistin, a last-resort antibiotic, is used to treat infections caused by multi-drug-resistant Gram-negative bacteria. Colistin resistance can emerge by acquiring the mobile colistin gene, <i>mcr-1</i>, usually plasmid borne. Studies on <i>mcr-1</i> and its transmissibility are limited in the Middle East and North Africa (MENA) region. Here, we investigated the occurrence of <i>mcr-1</i> in 18 previously collected <i>Escherichia coli</i> isolates collected from chicken samples in Qatar; whole-genome sequencing was performed to determine the location (plasmid-borne and chromosomal) of <i>mcr-1</i> in the isolates. Additionally, we assessed the transmissibility of plasmid-borne <i>mcr-1</i> and its cost on fitness in <i>E. coli</i> biofilms. Our results showed that the <i>E. coli</i> isolates belonged to different sequence types, indicating that <i>mcr-1</i> was occurring in strains with diverse genetic backgrounds. In silico analysis and transformation assays showed that all the isolates carried <i>mcr-1</i> on plasmids that were mainly IncI2 types. All the <i>mcr-1</i> plasmids were found to be transmissible by conjugation. In biofilms, a significant reduction in the number of CFU (≈0.055 logs CFU/mL) and colistin resistance (≈2.19 log CFU/mL) was observed; however, the reduction in resistance was significantly larger, indicating that the plasmids incur a high fitness cost. To our knowledge, this is the first study that investigates <i>mcr-1</i> transmissibility and persistence in Qatar. Our findings highlight that <i>mcr</i> has the potential to spread colistin resistance to potentially disparate strains and niches in Qatar, posing a risk that requires intervention.